1. Field of the Invention
The present invention relates to pigment granules and a process for the preparation thereof and their use.
2. Brief Description of the Prior Art
The processing of pigment granules requires milling of the pigments to give primary particles in order to achieve the optimum colour impression. The powders formed thereby produce a very large amount of dust and, owing to their finely divided nature, tend to adhere and stick in metering units. In the case of toxicologically hazardous substances, it is therefore necessary during processing to take measures for avoiding danger to man and environment from dusts which form. However, even in the case of safe inert substances, such as, for example, iron oxide pigments, avoidance of dust pollution is increasingly being desired by the market. Dust avoidance and improved metering based on good flow properties for achieving a qualitatively uniform colour impression on use in building materials and organic media is therefore the aim when handling pigments. This aim is achieved more or less by applying granulation processes to pigments. For example, pelletizing or spray granulation are used. In recent years, however, it has also been possible for briquetted and press granules increasingly to become established on the market. These granules are now very successful commercially.
In the case of pigments, the market requires in principle two conflicting properties when pigment granules are used: mechanical stability of the granule and good dispersing properties in the medium used. The mechanical stability is responsible both for good transport properties during transport between manufacturer and user and for good metering and flow properties during the use of the pigments. It is produced by strong adhesive forces and depends, for example, on the amount of binder or on the pressure during moulding by pressing. On the other hand, the dispersibility is influenced by thorough milling prior to granulation (wet and dry milling), by the mechanical energy during incorporation (shear forces) and by dispersants which immediately reduce the adhesive forces in the dry granules during incorporation into a medium. In the case of pigments, however, the use of larger amounts of dispersants is limited owing to the auxiliary/pigment cost ratio. Moreover, a high proportion of auxiliaries results in a corresponding reduction in the colour strength or scattering power. Since the colour strength variations are generally ±5%, for example when colouring building materials, the use of additives is also limited even if they simultaneously act as adhesion promoters and dispersants. Furthermore, the additives must not change the performance characteristics of the end products, such as, for example, building materials, plastics and finishes, in a disadvantageous manner, for example the compressive strength or the setting behaviour in the case of concrete, the compressive strength or abrasion resistance in the case of asphalt and the strength or the notched impact strength in the case of plastics, the elastic properties in the case of elastomers (polymers) and the rheological properties in the case of paints and finishes.
According to the prior art, for example, spray granulation (spray drying by means of a disc or nozzle) by the cocurrent or countercurrent method and pelletizing (mixer, fluidized-bed granulator, disc or drum) or compacting processes are suitable as production processes for pigment granules.
Granulation by spray drying starts from pigment suspensions with the use of binders. Appropriate processes are described in various patents. Water-soluble binders are predominantly used. Thus, organic substances, such as, for example, ligninsulphonates, formaldehyde condensates, gluconic acids and sulphated polyglycol ethers, are used as starting materials in DE 3 619 363 A1, EP 0 268 645 A1 and EP 0 365 046 A1, while inorganic salts, such as, for example, silicate and phosphate, are used as staring materials according to DE 3 918 694 A1 and U.S. Pat. No. 5,215,583 A1. A combination of spray granulation and pelletizing has also been described in EP 0 507 046 A1. In DE 3 619 363 A1 and EP 0 268 645 A1, the use of a compacting process is excluded.
EP 0 257 423 A1 and DE 3 841 848 A1 describe spray granulation with the use of polyorganosiloxanes as hydrophobic, lipophilic additives. The atomization drier mentioned generally leads to particle sizes which are too small and a large fine fraction. This means that a substantial proportion of the material from the drier is not obtained as directly usable granules but is first retained in the filter as a fine fraction and then has to be recycled to the process. The hydrophobing aftertreatment leads in the case of spray-granulated products to granules which are very free-flowing but produce an extremely large amount of dust.
EP 0 424 896 A1 discloses the preparation of low-dust fine granules in a production run in known intensive mixers. A low content of waxes in combination with emulsifier and wetting agents is used here by applying an aqueous dispersion. In general, water contents of 20 to more than 50% are obtained. These granules must first be dried and separated from oversize and undersize.
DE 31 32 303 A1 describes low-dust, flowable inorganic pigment granules which are mixed with binders becoming liquid under the action of heat (40 to 60° C.) and are granulated by a screen process with the use of a screening aid (pressure). About 10 to 20% of ihe throughput are obtained as a fine fraction of <0.1 mm.
EP 0 144 940 A1 discloses low-dust pigment granules which, starting from filter slurry, are mixed at 50 to 200° C. with about 50% of water by addition of 0.5-10% of surface-active substances and additionally mineral oil or waxes which become liquid, up to the smear point. The process takes place in intensive mixers, and possibly subsequent granulation and subsequent drying are also effected. Water is present in the end product in an amount of 10 to 15%, which is disadvantageous for incorporation into plastics.
Other processes, too, are limited in their application. Owing to drop formation, spray granulation requires the use of readily flowable, i.e. low-viscosity, suspensions. For the drying process, it is therefore necessary to evaporate a larger amount of water than in the case of the frequently usable fluidized-bed drying from highly pressed-out pigment filter pastes. This leads to higher energy costs. In the case of pigments prepared beforehand by calcination, spray granulation means an additional process step with high energy costs. Moreover, a larger or smaller proportion of fine material is obtained in the dust filter during the spray granulation which has to be recycled to the production.
DE 28 44 710 A1 describes the granulation of pigments in a fluidized bed with granulating auxiliaries, dry pigment powder being sprayed with water.
Pelletizing frequently also has disadvantages. Starting from pigment powder, it can be carried out in mixers with high turbulence, by the fluidized bed process or by disc and drum granulation. Common to all these processes is that the binder requirement, generally water, is high so that drying has to follow as an additional process step. Here too, granules of different sizes are obtained, particularly if insufficient binder is available for the amount of powder or the actual distribution is not optimum. A certain proportion of granules may then be too large, while on the other hand excessively small and therefore still dusting fractions are present. Classification of the granules formed with recycling of oversize and undersize is therefore required.
Disc granulation leads to a broad particle size spectrum of granules. Where this is undesirable owing to the poor dispersibility of particles which are too large, the granulation process has to be monitored by intensive monitoring by personnel and granule production has to be optimized by manual control of the amount of nuclei. Here too, classification with recycling of the oversize and undersize is usually effected.
DE 42 14 195 A1 discloses a process for colouring asphalt with inorganic pigment granules, in which oils are used as binders. This is a simple granulation process.
DE 196 38 042 A1 and DE 196 49 756 A1 describe inorganic pigment granules obtained from dry pigments, for example finished material, by mixing with one or more auxiliaries, compacting and further subsequent steps, such as comminution, screening and recycling of coarse and/or fine material. In the compacting step, compacting is effected with nip forces of 0.1 to 50 kN/cm. The granules obtained can be surrounded by an additional layer which serves for increasing the stability or as an aid in processing.
In DE 4 336 613 A1 and DE 4 336 612 A1 describe inorganic pigment granules obtained from dry pigments, for example finished material, by mixing with binders, compacting and further subsequent steps, such as crushing on a screen granulator and subsequent pelletizing on a rotating disc or in a rotating drum. In the compacting step, compacting is effected with nip forces of 0.1 to 15 kN/cm.
The granules prepared according to the teaching of DE 196 38 042 A1, DE 196 49 756 A1, DE 4 336 613 A1 and DE 4 336 612 A1 contain only the auxiliary or the auxiliaries which were added in the first process step to the pigment powder. Even if the granule particles are surrounded by an additional layer, they contain in their interior only the auxiliary or the auxiliaries which were added to the pigment powder in the first process step. The granule particles are composed in their interior of a homogeneous mixture of pigment and auxiliary or auxiliaries. However, it is known that auxiliaries which lead to very good dispersibility in an application medium on improvement of the product properties may be far less effective in another application medium, and in certain circumstances even incompatibility may be observed. Thus, for example, strongly hydrophobic auxiliaries may be advantageous in the case of incorporation into plastics or asphalt, while they lead to difficulties in the case of incorporation into aqueous emulsion paints or in the preparation of aqueous slurries, since the granules are only very poorly wetted with water. For this reason, the granules prepared according to the teaching of DE 19 649 756 A1, DE 4 336 613 A1 and DE 4 336 612 A1 are not simultaneously equally suitable for all application media. During the preparation, it would therefore have been necessary to add a plurality of auxiliaries which permit as good a processability as possible in all application media. This is not very expedient from the economic point of view and moreover the multiplicity of different additives may result in mutual incompatibilities. If granules are prepared from pigment mixtures according to the teaching of DE 19 649 756 A1, DE 4 336 613 A1 or DE 4 336 612 A1, a further disadvantage of these granulation processes is found. It is first necessary to prepare a mixture of the different pigments, which is then mixed with binders and other auxiliaries and further processed.
It was therefore an object of the present invention to provide a process which avoids the disadvantages described to date of compacting granulation when applied to inorganic or organic pigments and provides sufficiently stable, meterable, low-dust granules having good dispersibility in different application media.